1. A spontaneous process is one that occurs without external influence, driven by a decrease in energy or increase in disorder. 2. Enthalpy change alone does not determine spontaneity, as some endothermic reactions are spontaneous. Entropy must also be considered. 3. Entropy is a measure of disorder - solids have lowest entropy and gases highest. Spontaneous processes favor higher entropy states. 4. The second law of thermodynamics states that the entropy of the universe always increases for spontaneous processes, with ∆Suniverse > 0.

1. Chemical
thermodynamics

2. Spontaneous Process:
In Thermodynamics sense, a process that occurs by itself without the help of
external agent is a spontaneous process. All naturally occurring process is
spontaneous process.
Examples:
1. Rolling down of a stone downhill
2. Melting of ice
3. Flowing of water from higher to lower level
4. Vaporization of water
5. Burning of fuel
6. Mixing of gases
7. Natural radioactivity, etc.

3. Characteristics of Spontaneous Process:
1. All spontaneous process are unidirectional in nature and proceed
till the process is completed or a state of equilibrium is reached
2. Some spontaneous process can be reversed by applying external
agency.
3. Some spontaneous process requires initiations.

4. What is the driving force for spontaneous of a
process?
In the early development of thermodynamics energy change was considered
as criteria for spontaneity a process occurs with decrease in energy is
spontaneous. Example. flowing of water from higher to lower level, rolling
down of stone downhill etc. In case of chemical reaction, the reaction that
occurs with decrease in enthalpy i.e. exothermic is spontaneous like burning
of Methane, neutralization reaction, etc. On the basis of enthalpy change,
∆H = -ve = exothermic = spontaneous
∆H = +ve = endothermic = non-spontaneous
∆ H = O = equilibrium

5. However many endothermic reaction were found to be spontaneous. Example,
i. Melting of ice;
H2O (s) → H2O (l) ∆H = +6 KJ
ii. Vaporization of water
H2O (l) → H2O (g) ∆H = +40.8 KJ
iii. Dissociation of NH4NO3 in water,
NH4NO3 (s) + H2O (l) → NH4
+ (aq) + NO3
– (aq) ∆H = +25KJ
These examples show that enthalpy change alone is not an enough criteria for spontaneity.
These lead to necessity of another thermodynamic term.
In all above examples, the molecular disorderness is increased, so the new
thermodynamic function should measure disorderness. This function is called entropy.

6. Entropy:
Entropy of a system is the measurement of disorderness or randomness of
the system. The greater the randomness, the higher is the entropy. For a
given substance, the crystalline solid state has lowest entropy i.e. in solid,
the molecules are arranged orderly so it has least entropy but in liquid the
disorderness is more than solid while in gas, the disorderness is maximum.
So the order of entropy of a substance in different physical states, is,
Ssolid < Sliquid << Sgas
Entropy is a state function and depends only upon initial and final states, also
change in entropy ( ∆s) will be
Δ S = S2 – S1
Where S2 = Final state entropy
S1 = Entropy of Initial state

7. In chemical reaction, the change in entropy is,
Δ Sreaction = Sproducts – Sreactants
= qrev, iso/T…………………..(i)
( simply Δ Sreaction = q/T)
i.e. entropy change during the process is defined as the amount of heat
(q) absorbed isothermally and reversibly (infinitesimally slowly) divided
by the absolute temperature (T)

8. Justifications:
1. When a system absorbs heat, the molecule start moving faster
because kinetic energy increases. Hence the disorder increases.
More the heat absorbed, greater is the disorder.
2. For the same amount of heat absorbed at low temperature, the
disorder is more than at higher temperature. Thus the presence of T
in the denominator is justified.
Unit
Its unit is Jmol-1K-1

9. Limitation of the first law of thermodynamics.
1. First law does not provide a clear idea about the direction of
absorption or evolution of heat.
2. The information provided by the first law of thermodynamics are
not enough to predict the spontaneity or feasibility of a process.
To deal with spontaneity of the chemical process some new terms
viz., entropy and Gibbs free energy were introduced in
thermodynamics which lead to the formulation of second law of the
thermodynamics.

10. Second law of thermodynamics:
There are limitation of first law of thermodynamics. To overcome those
limitation and to explain the spontaneity of a process, second law of
thermodynamics is stated. It state that,
“ Whenever a spontaneous process takes place, it is accompanied by an
increase in the total entropy of universe.” Thus for a spontaneous process
∆Suniverse = ∆Ssys + ∆Ssurr > 0
According to second law, when an irreversible spontaneous process occurs,
the entropy of the system and surrounding increases. i.e. ∆Suniverse>0.
As we consider entire universe as a system, the second law can be stated as,
“ The entropy of the universe is constantly increasing.”